Move quaddtype into its own repository

[ngoldbaum]

Over in #227 (comment), I suggested creating a new repository for quaddtype. Currently most of the activity in this repository is dominated by quaddtype and IMO it's outgrown this repository. It's also possibly confusing or discouraging for anyone who wants to experiment with adding a new dtype.

To do this, we need to:

• create a new repository in the NumPy org. It should preserve all the history in the quaddtype folder as well as the CI configuration.
• Transfer issues and pull requests related to quaddtype to the new repository
• Update CI configuration to only test quaddtype
• Update documentation and downstream links like the conda-forge feedstock to point to the new repository.

[ngoldbaum]

I asked at today's triage meeting if it's OK to create a new repo and no one objected.